Attorney General Eric Holder signed off on a controversial search warrant that identified Fox News reporter James Rosen as a "possible co-conspirator" in violations of the Espionage Act and authorized seizure of his private emails, a law enforcement official told NBC News on Thursday.

The disclosure of the attorney general's role came as President Barack Obama, in a major speech on his counterterrorism policy, said Holder had agreed to review Justice Department guidelines governing investigations that involve journalists.

"I am troubled by the possibility that leak investigations may chill the investigative journalism that holds government accountable," Obama said. "Journalists should not be at legal risk for doing their jobs."

Rosen, who has not been charged in the case, was nonetheless the target of a search warrant that enabled Justice Department investigators to secretly seize his private emails after an FBI agent said he had "asked, solicited and encouraged ... (a source) to disclose sensitive United States internal documents and intelligence information."

Obama's comments follow a firestorm of criticism that has erupted over disclosures that in separate investigations of leaks of classified information, the Justice Department had obtained private emails that Rosen exchanged with a source and the phone records of Associated Press reporters.